In the fabrication of apparatus for transferring heat and/or mass that contain a relatively large number of tubes, tube sheets are usually used that have through-holes into which the tube ends are threaded and connected with the tube sheet. Various processes to arrange the tube ends as compactly as possible, especially in the tube sheet zone, have been developed. The closer together the tube ends are arranged, the more difficult it becomes to connect the tube ends with the tube sheet in such a manner that a leaktight connection is ensured even under relatively high pressures.
For leaktight connection of tube ends that are arranged as compactly as possible in heat exchangers containing tubes having no or only little porosity, a process has been developed in which the tube ends are placed tightly against each other and are connected to each other by radiant heat (West German Laidopen application Ser. No. 3,614,339). For somewhat less compactly arranged tube ends for apparatuses for transferring heat and/or mass, a connection between the tube ends can also be achieved by means of radiant heat by threading each tube end into a sleeve and placing the sleeves close together (West German Laid-open application Ser. No. 3,614,342).
Especially for the latter apparatus for transferring heat and/or mass (West German Laid-open application Ser. No. 3,614,342), very much positioning work is necessary in the fabrication thereof, because both the sleeves and tube ends must be held in position during the radiant-heat treatment. For better handling, it would be favorable first to connect all sleeves with each other before the tube ends are positioned in the connected sleeves. It would be even more favorable to be able to fabricate a molded shape that resembles sleeves connected with each other. In this case, however, care is to be taken that the minimum web width between neighboring holes of the tube sheet, which width in the case of circular holes lies on an imaginary connecting line between the centers of these neighboring holes, corresponds in order of magnitude to the wall thickness of the tube ends, in order to achieve uniform melting of tube sheet and tube ends during heat transfer by radiant heat. A leaktight connection between tube sheet and tube ends is achieved only by uniform melting of tube sheet and tube ends.
Since tubes with an outside diameter of 3 mm and a wall thickness of 0.4 mm or less are usually used in such apparatus for transferring heat and/or mass, the minimum web width between neighboring holes must also be at most 0.4 mm. Especially when 25 tubes or more (up to several thousand) are used in a single apparatus for transferring heat and/or mass, it has been impossible heretofore to fabricate molded shape with such small minimum web widths and such small cross-sectional dimensions for the through-holes from thermoplastic material using injection molding.
Injection molding is known to relate to processes in which thermoplastic material is introduced into an injection mold and cooled under pressure. The needles necessary in the injection mold to obtain the through-holes are not stable, because of the small cross-sectional dimension (3 mm diameter or less), and they deform because of the melt flowing between them, so that the maintenance of exact web widths is not possible and the holes even coalesce to some extent. Especially for tube sheets with 25 holes and more, it cannot be guaranteed that the melt will completely fill the injection mold in the case of these small web widths.